The Hard X-ray Imager onboard Hitomi (ASTRO-H)
Kazuhiro Nakazawa,1 Goro Sato,2 Motohide Kokubun,2 Teruaki Enoto,3 Yasushi Fukazawa,4 Kouichi Hagino,2Atsushi Harayama,2 Katsuhiro Hayashi,5 Jun Kataoka,6 Junichiro Katsuta,4 Philippe Laurent,7 Francois Lebrun,7Olivier Limousin,7 Kazuo Makishima,8 Tsunefumi Mizuno,9 Kunishiro Mori,2 Takeshi Nakamori,10 Toshio Nakano,11Hirofumi Noda,12 Hirokazu Odaka,13 Masanori Ohno,4 Masayuki Ohta,2 Shinya Saito,14 Rie Sato,2 Hiroyasu Tajima,15Hiromitsu Takahashi,4 Tadayuki Takahashi,2 Shin’ichiro Takeda,16 Yukikatsu Terada,17 Hideki Uchiyama,18Yasunobu Uchiyama,14 Shin Watanabe,2 Kazutaka Yamaoka,15 Yoichi Yatsu,19 Takayuki Yuasa,8 and the HXI team1

Department of Physics, University of Tokyo, Tokyo 113-0033, Japan
2
ISAS/JAXA, Sagamihara, Kanagawa 252-5210, Japan
3
Faculty of Science, Kyoto University, Kyoto 606-8502, Japan
4
Department of Physical Science, Hiroshima University, Hiroshima 739-8526, Japan
5
Division of Particle Physics and Astronomy, Nagoya University, Aichi 464-8602, Japan
6
Research Institute for Science and Engineering, Waseda University, Tokyo 169-8555, Japan
7
APC/ IRFU / Service d’Astrophysique, CEA Saclay, 91191 Gif-sur-Yvette Cedex, France
8
MAXI team, Global Research Cluster, RIKEN, Saitama 351-0198, Japan
9
Hiroshima Astrophysical Science Center, Hiroshima University, Hiroshima 739-8526, Japan10
Department of Physics, Yamagata University, Yamagata 990-8560, Japan
11
Nishina Center, RIKEN, Saitama 351-0198, Japan
12
The Frontier Research Institute for Interdisciplinary Sciences, Tohoku University, Sendai 980-8578, Japan
13
SLAC, National Accelerator Laboratory, Stanford University, California 94025-7015, USA
14
Department of Physics, Rikkyo University, Tokyo, 171-8501, Japan
15
Institute for Space-Earth Environmental Research, Nagoya University, Aichi 464-8601, Japan
16
Advanced Medical Instrumentation Unit, OIST, Okinawa, 904-0495 Japan17
Department of Physics, Saitama University, Saitama 338-8570, Japan
18
Science Education, Faculty of Education, Shizuoka University, Shizuoka, 422-8529, Japan
19
Department of Physics, Tokyo Institute of Technology, Tokyo 152-8551, Japan
E-mail(FS): nakazawa@juno.phys.s.u-tokyo.ac.jp
Abstract
The hard X-ray imager (HXI) onboard Hitomi (ASTRO-H) mission was coupled with the hard X-ray
telescope (HXT) to provide fine imaging spectroscopy at 5 to 80 keV. The system is made of 2 sets ofHXI+HXT, covering a field-of-view of 9×9 armin2 . Hitomi was launched in 17 February 2016 and the HXIstarted observation on 14 March, and was operational till the incident lead to Hitomi loss on 26 March. All
detector channels worked well and showed performance consistent with those seen on ground. Calibration
observations confirmed the 5–80 keV energy coverage, with eﬀective area meeting the requirement. The
background level was shown to be as low as 1–3×10−4 counts s−1 keV−1 cm−2 (here cm2 is in detector
geometrical area), also meeting the requirement. These properties showed that the HXI was one of themost sensitive instruments in 10-80 keV.
Key words: Hitomi, Hard X-ray Imager, DSSD, CdTe-DSD, BGO, in-orbit performance

1. INTRODUCTION
Hitomi X-ray observatory (e.g. Takahashi et al., 2016)
launched in 17 February 2016 had a hard X-ray imaging
spectroscopy system made of two hard X-ray imagers(HXIs, Nakazawa et al., 2016) coupled with two hard Xray telescopes (HXTs, Awaki et al. 2016). With 12 m
focal length, they provide fine (1′ .7 half-power diame-ter) imaging spectroscopy at 5 to 80 keV. They are designed to provide crucial information about hottest thermal emission, non-thermal emission, as well as reflection
components of high-energy astrophysical objects.
Specification of the HXI+HXT system is summarizedon Table 1 and Figure 1. Two HXIs are mounted on
the HXI plate, connected to the satellite baseplate via

100

Effective Area (cm2)

NuSTAR FPMA
HXI-1 after layer selection

4 arcmin radius integrated
effective area
5102050100

Energy (keV)
280 arcmin scaled background
NuSTAR FPM

HXI-1 after layer selection
10 4the Extensible Optical Bench (EOB). The EOB deploysby 6 m after launch. On top of the satellite, the Fixed
Optical Bench is mounted and the two HXTs are locatedon its top-plate. With this configuration, the 12 m focal
length of the HXTs is achieved.
The HXI imagers are made of 4 layers of Si and aCdTe semiconductor double-sided strip detectors, with32 × 32 mm size, stacked to enhance detection eﬃciency
as well as to enable photon interaction-depth sensing,valid for background reduction (G. Sato et al. 2016).
Active shield made of 9 BGO scintillators surrounds theimager to provide low background environment. Afterthe EOB deployment on 28 February, the HXI was gradually turned on. Two imagers successfully started observation on 14 March, and was operational till 26 March.

HXI-1 total

10Fig. 1. (left) The hard X-ray imaging spectroscopy system of Hitomi. The 2 HXI-Ss are located at the HXI plate, connected tothe satellite main body via the EOB. (right) Photo of the HXIplate equipped with 2 HXI-Ss covered with thermal blanket using
Ag-Teflon layer at the surface. Also seen in front is the radiator.

0.1

2x HXIsHXI plate

0.01

Extensible Optical
Bench (+6m)

10 3

12 m
focal length

sitive to ∼ 100 keV electrons in orbit, after proper data
selection, the background level was shown to be as low
as 1–3×10−4 counts s−1 keV−1 cm−2 (here cm2 is indetector geometrical area). As shown in Figure 2, theHXI background is lower, and the eﬀective area is higher
than those of NuSTAR. Combined with ∼ 2 times worse
angular resolution, we can see that the statistical sensitivity limit is similar for point source in the two missions.
For largely extended sources, the HXI+HXT’s statisticallimit is twice better. Because of the nearly perfect CXB
baﬄing, which could not be achieved in NuSTAR, theHXI has more stable background and better potentialfor dim extended sources, as well.

normalized counts s 1 keV 1

2x HXTs5102050

Energy (keV)

Table 1. Specification of the HXI+HXT system (2 systems added).

angular resolution
FoV
eﬀective area
energy band

1′ .7@30 keV HPD
9′ .17 × 9′ .17 (32 × 32 mm)> 300 cm2 @ 30 keV total
5–80 keV

2. In-orbit performance
After the commissioning operation, we first verified all
1280 ch each of the camera and 9+2 ch each of the APDs
were working well. Hitomi observed a pulsar wind nebula G21.5−0.9 as the first light for the HXI. Then, itobserved an isolated neutron star RXJ1856−3754 forsoft X-ray instruments. Then, Hitomi observed Crabfor 4 orbits, i.e. ∼6 hours in duration. The Crab andG21.5−0.9 spectra verified the eﬀective area of the system (see Awaki et al. 2016).
The RXJ1856−3754 data provided us with the background data. Although the HXI turned out to be sen-Fig. 2. (left) Preliminary eﬀective area accumulated over a radius of4′ of the HXI1-S. Also shown in black in comparison is that ofthe NuSTAR FPMA. (right) Preliminary background level of theHXI1-S, after the layer selection accumulated over 80 arcmin2 area(magenta), equivalent to 9.9 cm2 geometrical area. Also shown(in black) in comparison is that of the NuSTAR FPM, scaled fromthe NuSTAR proposal information (6.9 cm2 geometrical).

References
Takahashi, T. and Hitomi-collaboration, “The astro-h xray astronomy satellite”, Proc .SPIE 9905 (2016).
Awaki, H. et al., “Performance of astro-h hard x-ray
telescope (hxt)”, Proc .SPIE 9905 (2016).
Nakazawa, K. et al., “The Hard X-ray Imager onboard
ASTRO-H”, Proc .SPIE 9905 (2016).
Sato, G. et al., “The Si/CdTe semiconductor cameraof the ASTRO-H Hard X-ray Imager (HXI)”, NIM-A
831, p.235–241 (2016).